Melt the butter in a heavy 4-quart saucepan over medium heat. Add the onion and garlic and cook, stirring occasionally, until translucent, slightly golden, and soft, about 10 minutes. Whisk in the remaining ingredients and simmer, stirring occasionally, until the sauce thickens, 35 to 45 minutes. Taste the sauce and adjust the salt and pepper.

Let the sauce cool until it’s warm but no longer hot, about 10 minutes. Purée in a blender or with an immersion blender. Let the sauce cool and pour it into a clean glass jar for storing. The sauce can be made ahead and kept for up to 1 week in the refrigerator.